Beginning in 1972, the Congress of the United States instituted a comprehensive regulatory scheme for tracking and reducing water pollution. These provisions, known generally as the Clean Water Act, have become progressively more restrictive in the type and amount of pollutants facilities are permitted to discharge. The facility (point source) is controlled by regulations citing technology-based effluent limitations. For toxic and non-conventional pollutants the facility must use the best available technology economically achievable and the effluent limitations are reviewed every five years.
One area of particular concern for both industry and for government regulators is the release of heavy metals into the environment. Heavy metals such as cadmium, lead and zinc have been found to be toxic in minute quantities. The Environmental Protection Agency suggests that it takes an unstressed ecological system approximately three years to recover from a pollution event in which excessive amounts of cadmium are released.
Lead has been pin-pointed as an especially dangerous heavy metal. It tends to concentrate in the skeletal system of humans and animals, amplifying its toxicity as the individual is exposed over time. Elevated levels of lead in blood samples taken from children in urban areas are particularly alarming. Lead has been linked to cancer development, brain damage and birth defects.
Organic chemicals of various sorts such as carbon tetrachloride, chloroform, trichloroethane, tetrachloroethane, perchloroethylene, trichloroethylene, phenol, benzene, dichlorobenzene and many others are commonly used in manufacturing processes and are frequently found in wastewater streams. As in the case of heavy metals, the discharge regulations have grown progressively more stringent, allowing smaller amounts of these pollutants to be discharged into the environment. American industry has been forced to treat effluent containing these compounds to remove all but trace amounts, often at considerable expense.
The electronic and defense industries frequently generate waste streams containing both organic chemicals and heavy metals. Metal finishing, stripping and painting operations produce wastewaters contaminated with phenol, tetrachloroethylene, trichloroethylene, ethylene chloride, isopropyl alcohol, and ketones in conjunction with one or more heavy metals such as lead, cadmium and others. The aqueous-phase concentrations of organics may range from ten to several hundred ppm as in the case of phenol and from one to five hundred ppm lead.
There have been substantial efforts directed to finding cost effective methods to remove these heavy metals and organic materials from wastewaters. Many of the methods currently in use or under study involve the adsorption or absorption of the organics onto a treatment matrix. Activated carbon is commonly used in wastewater treatment facilities to remove organic contaminants from wastewater streams. Peroxide oxidation is also a common treatment technique for removal of organic contaminants.
Wastewater contaminated by heavy metals is typically treated by reducing processes, coagulation, precipitation or a combination of these techniques. These procedures are both time-consuming and expensive. While adsorption onto activated carbon is commonly used to treat wastewater to remove organics, the use of adsorbents for treating the heavy metals in wastewater is still in the experimental stage. Numerous investigators have demonstrated that activated carbon can be used to remove heavy metals, at least at the laboratory scale. Activated carbon is, however, expensive.
Heavy metals and hazardous organic materials are also sometimes found in the runoff and groundwater associated with contaminated industrial sites and landfills. Many such sites are abandoned, leaving contaminants which may migrate with natural water flows from the site. Such contaminated water flows could pose a danger to human health and the environment, forcing industry and government to intervene to protect the public interest. A similar situation exists in disposal sites where contaminants were inadequately contained.
There is a need for an economical method of intercepting contaminated water flows and fixing the contaminants in situ. There exists, additionally, a need in industry for an easily applied, cost-effective sorbent to remove heavy metal and organic contaminants from contaminated water streams.